Update provider documentation

[fewieden]

I was reading through the documentation in order to figure out how to add a new provider.

I found two places that refer to a big change https://github.com/vercel/commerce/pull/252:

• https://github.com/vercel/commerce/blob/main/packages/commerce/new-provider.md#adding-a-new-commerce-provider
• https://github.com/vercel/commerce/blob/main/packages/commerce/README.md#commerce-api

Since the PR was already merged in June last year the documentation should be updated.

Are only the references to the PR outdated or does the whole integration documentation need an update?